In the reproduction of video signals, particularly color television signals, from a record carrier, various errors appear in the signals which need to be compensated to a considerable extent for obtaining the best possible picture reproduction. Most important are time and tape speed errors and also signal dropouts resulting from localized defects of the tape or other record carrier.
It is known to store picture signals in a picture memory which are obtained segment by segment (each segment being an oblique track on a tape) in order to be able to make a "stop frame" still picture reproduction.
Various arrangements have become known, furthermore, for dealing with the types of errors above noted, operating either on an analog or a digital base.
One known system for compensation of time errors in color television signals obtained from a tape record is described in my U.S. Pat. No. 4,376,291. In that system, the color television signals are converted into digital signals and then stored in digital form. First clock signals (C1) are generated the phasing of which is dependent upon the horizontal synchronization signals contained in the color television signal picked up from the tape and the frequency of which is controlled by a first control voltage. The first clock signals (C1) are used for analog-to-digital conversion and for writing the digital signals in a first intermediate memory. A second clock signal (C2), the frequency of which is an integral multiple of a horizontal scanning rate reference signal is used for reading the digital signals out of the first intermediate memory. The deviation of a line period of the digital signals read out of the first intermediate memory from the line period of the reference signal is measured for producing a first control voltage. The horizontal scanning frequency pulses of the reference signal are compared with the corresponding pulses of the signals read out of the first intermediate memory with reference to relative phase and, in a manner dependent upon the phase difference thus found, the writing and reading of digital signals respectively in and out of a memory is so controlled that the time between writing in and reading out corresponds to the phase difference. The signals read out of the memory are later written into a second intermediate memory, from which they are read out by means of third clock signals (C3) which are obtained by controllable phase shifting of the second clock signals. A phase comparison is produced between the color synchronizing signal of the signals read out of the second intermediate memory and a reference color synchronizing signal and the result of this comparison for each line is stored for obtaining a second control signal which is utilized for phase shift of the second clock signal.
A digital picture memory for producing stop frame still pictures or speeded up or slow-motion pictures of the recorded scenes can be connected to follow the equipment utilized in the above-described method.
The compensation of the above-described errors in the known method above described leaves considerable room for improvement. It is an object of the present invention to improve such compensation in a manner as effective as possible at reasonable costs.
Looking more closely at the problem of tape speed errors, particularly fluctuations in speed resulting from various mechanical tolerances in the manufacture of the recording and reproduction machinery handling the tape resulting in what is known as "time base errors", for want of more suitable time marks in the video signal it has been common to correct time base errors only at the end or at the beginning of each line, for example by a phase comparison of the horizontal synchronization pulses of the reproduced signal with reference pulses of the horizontal scanning frequency. In known methods of making this correction the video signals are subjected to a delay which is controlled in a manner tending to correct the timing errors.
In other known systems the video signals are written into a memory with clock pulses subject to timing errors which are derived from the recorded signals and then read out with clock pulses that are free of timing errors. Even with these timing error compensators the timing errors can be measured merely at the end or at the beginning of the lines. Some methods have become known also for compensating the change of timing errors within a line, these changes being referred to as speed or velocity errors. In these methods the delay of the video signals serving for compensation of the timing errors is gradually changed during the running of a line in the sense of the timing error to be expected at the beginning of the next line. For this purpose the known methods compare the timing errors in successive horizontal blanking intervals with each other.
The further processing of the velocity errors determined by this comparison is based on the particular kind of recording method used. In tape machines operating with transverse recording tracks that run almost perpendicular to the tape edges and using four magnetic heads which record successive tracks in turn, and also in the case of the so-called oblique track method producing longer segments as tracks, velocity error compensators are used in which error signals are obtained over a number of head wheel revolutions relating to the velocity errors of lines and for each of the magnetic heads which always have the same position on the tape relative to the tape edge. It has become known mainly in connection with other oblique track recording methods to measure the velocity error every time for a line and to delay the video signals by one line, so that the error signal can be used for the very same line.
Both of the above-described methods of dealing with velocity errors have disadvantages, particularly in connection with machines that operate on the oblique track segment recording method. It is accordingly another object of the present invention to improve particularly the compensation of velocity errors, namely the compensation of the change of timing errors within each line.
It is a further object of the invention to provide a similar velocity error compensation for the color carrier synchronizing signal.
In analog-to-digital conversion of video signals, particularly color television signals, it can be advantageous for various reasons to establish a coupling between the clock signal for the conversion and the video signal. Methods have become known in which the video signal is written into a memory after an analog-to digital conversion in which the clocking or sampling for the conversion and writing the digital signals into a memory is subject to the same timing errors as the video signal. For proper operation of such systems it is necessary to have precise phase coupling of the sampling signal with the reproduced video signal, particularly with the digitized video signal as it appears. It is a further object of the invention to improve the precision of this coupling of the clock signals with the video signal as obtained from the tape.
Another problem in color television signal processing lies in the fact that in magnetic tape recording and reproduction systems in which relatively long oblique tracks known as "segments" are recorded on the tape, many such segments or tracks need to be recorded for one picture field and it is necessary to switch from one magnetic head to another many times in a picture field. In known recording devices this switching is performed during the blanking interval, as the result of which the horizontal synchronization pulses are lost during the switch-over. This loss does not interfere with reproduction of the signals on a monitor or radiation of the signals by a broadcasting station, since in known equipment for those purposes new synchronizing signals are supplied to the television signals.
For detecting and compensating timing errors, however, it is necessary to detect the horizontal synchronization pulses, to measure their time deviation from reference pulses and accordingly to correct the time position of the television signals. The same applies for velocity errors which in the past have been compensated for by slightly time-expanding or time-compressing the portions of lines which incorporate the picture content.
In the known recording and reproduction apparatus, however, there is no horizontal synchronization pulse available at the beginning of the first line after the switch-over between magnetic heads. In the known apparatus extrapolation of the values for the other lines is applied to the first line. It is a further object of the present invention to provide for directly determining the timing error for the first line after a switchover from one head to another, as an optional further improvement in the processing of color television signals.
Another improvement that is desirable relates to handling signal dropouts which provide a disturbing effect in the picture as the result of defects in the magnetizable layer of the record tape. It has long been known to determine the occurrence of such dropouts by suitable circuits and to replace the missing signal portion by signal portions from preceding lines. In the case of color television signals it is important in such case to replace the chrominance signal with the correct color carrier phase, particularly in systems like the PAL system in which there is a phase reversal between alternate lines.
It has been found useful in the past to split the color television signals into chrominance and luminance signals before treatment by the compensation circuit and then, upon detection of a signal dropout, to delay the luminance signal by one line and the PAL color signal by two lines.
These known methods of processing were first carried out with analog circuits and later with digital circuits. In the case of digital circuits for error correction of color television signal which also include circuits for compensation of signal dropouts, a quite high quantization precision, requiring the resolution provided by nine-bit samples, for example, is necessary in order to meet all the requirements that are imposed in professional (studio) television standards. This high bit "width" magnifies the expense of the various digital circuits to be used, however. Furthermore a supplemental signal path is necessary in the known arrangement for the color television signals not split up into luminance and chrominance, so that the split and recombined signals may be used only when there is a signal dropout.
It is accordingly another object of the invention to provide also a more economically practicable method of compensating signal dropouts in color television signals reproduced from a magnetic tape or the like.